May 22, 2013

Study: Allosaurus Ate Less Like A Crocodile, More Like A Falcon

To dismember its prey, the mighty Tyrannosaurus rex thrashed its massive head from side to side. T. rex´s smaller cousin, the Allosaurus, was a more dexterous hunter and tugged at prey more like a modern day falcon, a new study from Ohio University shows.

"Apparently one size doesn't fit all when it comes to dinosaur feeding styles," said Ohio University paleontologist Eric Snively. "Many people think of Allosaurus as a smaller and earlier version of T. rex, but our engineering analyses show that they were very different predators."

A diverse team of Ohio University researchers, including experts in mechanical engineering, computer visualization and dinosaur anatomy, were led by Snively in analyzing a high-resolution cast of the five-foot-long skull plus neck of the 150-million-year-old predatory therapod, the Allosaurus. Fossils at the O´Bleness Memorial Hospital in Athens were CT-scanned to produce digital data that the authors could manipulate in a computer. Their findings were reported online in the journal Palaeontologia Electronica.

Snively collaborated with John Cotton, a mechanical engineer, to apply a specialized engineering analysis borrowed from robotics called “multibody dynamics.” This analysis allowed the researchers to run sophisticated simulations of the head and neck movements Allosaurus made when attacking prey, stripping flesh from a carcass or even just looking around.

"The engineering approach combines all the biological data–things like where the muscle forces attach and where the joints stop motion–into a single model. We can then simulate the physics and predict what Allosaurus was actually capable of doing," said Cotton, an assistant professor in the Russ College of Engineering and Technology.

The scientists had to “re-flesh” Allosaurus to figure out how the dinosaur de-fleshed a Stegosaurus. They used the anatomical structure of modern-day dinosaur relatives like birds and crocodilians combined with telltale clues on the dinosaur bones to build in neck and jaw muscles, air sinuses, the windpipe and other soft tissues into their Allosaurus 3D computer model.

"Dinosaur bones simply aren't enough," said Lawrence Witmer, Chang Professor of Paleontology in the Heritage College of Osteopathic Medicine. Witmer is the principal investigator on the National Science Foundation's (NSF) Visible Interactive Dinosaur Project that provided funding for this research. "We need to know about the other tissues that bring the skeleton to life."

The longissimus capitis superficialis is an unusually placed neck muscle that provided a key finding for the researchers. In previous studies, Snively noticed that in most predatory dinosaurs, such as T. rex, this muscle passed from the side of the neck to a bony wing on the outer back corners of the skull.

"This neck muscle acts like a rider pulling on the reins of a horse's bridle," explained Snively. "If the muscle on one side contracts, it would turn the head in that direction, but if the muscles on both sides pull, it pulls the head straight back."

The analysis of Allosaurus, however, revealed that the logissimus muscle attached much lower on the skull, which would have cause “head ventroflexion followed by retraction,” according to the engineering analysis.

"Allosaurus was uniquely equipped to drive its head down into prey, hold it there, and then pull the head straight up and back with the neck and body, tearing flesh from the carcass “¦ kind of like how a power shovel or backhoe rips into the ground," Snively said.

Small falcons, such as kestrels, use this same de-fleshing technique, while Tyrannosaurs like T. rex were engineered to use a grab-and-shake technique to tear off hunks of flesh like a modern day crocodile.

There is a cost to T. rex´s feeding style, according to the research team: high rotational inertia. The T. rex´s large bony and toothy skull made it hard to speed up or slow down its head or to change its course as it swung its head around. On the other hand, as the team restored the soft tissues and air sinuses, they discovered that Allosaurus had a relatively light head.

Rotational inertia is increased by having a lot of mass sitting far away from the axis of head turning, such as with the T. rex. Allosaurus´ lighter head decreases rotational inertia. You can think of this like an ice skater spinning faster and faster as she tucks her arms and legs into her body, decreasing her rotational inertia as the mass of her limbs moves closer to the axis of spinning.

"Allosaurus, with its lighter head and neck, was like a skater who starts spinning with her arms tucked in," said Snively, "whereas T. rex, with its massive head and neck and heavy teeth out front, was more like the skater with her arms fully extended “¦ and holding bowling balls in her hands. She and the T. rex need a lot more muscle force to get going."

This means that Allosaurus was a much more flexible hunter, able to move its head and neck around relatively rapidly and with considerable control. The cost of that control, however, was a loss of brute-force power that required a de-fleshing style that recruited the entire neck and body to strip flesh from the bones, much like a modern falcon.

The team intends to continue their research, using an engineering approach to explore additional differences in dinosaur feeding styles.